Recently, wireless mesh networks attract more and more attention, e.g. for remote control of illumination systems, building automation, monitoring applications, sensor systems and medical applications. In particular, a remote management of outdoor luminaires, so-called telemanagement, becomes increasingly important. On the one hand, this is driven by environmental concerns, since remote control systems or so-called telemanagement systems enable the use of different dimming patterns, for instance as a function of time, weather conditions and season, allowing a more energy-efficient use of the outdoor lighting system. On the other hand, this is also driven by economical reasons, since the increased energy efficiency also reduces operational costs. Moreover, the system can remotely monitor power usage and detect lamp failures, which allows for determining the best time for repairing luminaires or replacing lamps.
Current radio-frequency (RF) based wireless solutions use either a star network topology or a mesh network topology. In a star network, a data collector has a direct communication path to every node in the network. However, this typically requires a high-power/high-sensitivity base-station-like controller placed at a high location (e.g. on top of a building), which makes the solution cumbersome to deploy and expensive. In a mesh network, the plurality of nodes does in general not communicate directly with the controller, but via so-called multi-hop communications. In a multi-hop communication, a data packet is transmitted from a sender node to a destination node via one or more intermediate nodes. Nodes act as routers to transmit data packets from neighboring nodes to nodes that are too far away to reach in a single hop, resulting in a network that can span larger distances. By breaking long distances in a series of shorter hops, signal strength is sustained. Consequently, routing is performed by all nodes of a mesh network, deciding to which neighboring node the data packet is to be sent. Hence, a mesh network is a very robust and stable network with high connectivity and thus high redundancy and reliability.
In the prior art, mesh network transmission techniques can be divided in two groups: flooding-based and routing-based mesh networks. In a flooding-based mesh network, all data packets are forwarded by all nodes in the network. Therefore, a node does not have to make complicated routing decisions, but just broadcasts the data packet. By these means, the technique is quite robust. However, in large networks, the data overhead due to forwarding impacts the overall data rate. Moreover, collisions of data packets are more likely to occur, further reducing the overall performance. Hence, the main problem of this solution is the scalability. Routing-based mesh networks can be further divided into proactive and reactive schemes. In proactive routing-based mesh networks, all needed network paths are stored in routing tables in each node. The routing tables are kept up to date, e.g. by sending regular beacon messages to neighboring nodes to discover efficient routing paths. Although the data transmission is very efficient in such kind of network, the scalability is still low, since in big networks, the proactive update of the routing tables consumes large parts of network resources. Moreover, the routing tables will grow with the scale of the network. In addition, the setup of the network requires time and resources in order to build up the routing tables. Reactive schemes, in contrast, avoid the permanent overhead and large routing tables by discovering routes on demand. They use flooding to discover network paths and cache active routes or nodes. When routes are only used scarcely for single data packets, flooding the data packets instead of performing a route discovery might be more efficient. If routes are kept long enough to avoid frequent routing, reactive schemes degenerate to proactive schemes. An example for a reactive routing-based mesh network is used in ZigBee. However, the main problem of this protocol scheme is still the scalability of the network.
In large-scale multi-hop networks, the number of hops a data packet has to travel is large as compared to a hop distance in small networks. In a large radio frequency telemanagement system comprising thousands of nodes, 20-40 hops are likely to occur. However, the delivery chance of an individual data packet decreases with its hop distance, since with every hop, there is a chance that the data packet gets lost.
Hence, a big disadvantage in common wireless mesh networks is constituted by the very limited network scalability. This is due to the fact that every data packet or message is transmitted multiple times due to the forwarding, whereby the overall network throughput is reduced. Also, data packet collisions are more likely to occur causing data packet losses, further reducing the overall performance. Thus, improving the success and reliability of multi-hop end-to-end transmissions is particularly crucial in large-scale multi-hop networks, such as street illumination systems with a high number of luminaire nodes, since end-to-end retransmissions are far more resource/bandwidth costly and delay intensive than in typical smaller networks. Hence, efficient routing protocols and reduction of end-to-end delays are required for large-scale wireless mesh networks in order to achieve the required throughput, response times and robustness. Moreover, when a data packet is dropped during the final hops to its destination, it has to be retransmitted by its sender node. This causes large delays as well as delay differences in the communication between any two nodes in the network, leading to a poor user experience due to the high and/or heterogeneous delays, e.g. when interacting with the luminaire nodes of an illumination system.
In order to determine whether a data packet is successfully delivered or got lost, data packet transmissions are commonly performed in acknowledgement mode. In a hop-by-hop acknowledgement mode, every hop of the multi-hop transmission is confirmed by the receiving node to the preceding transmitting node. However, this leads to high network load. Thus, often end-to-end acknowledgements are used, wherein the final destination node confirms the receipt of the data packet to the initial sender node. In this mode, the sender node waits for a predetermined time, so-called acknowledgement time-out, before retransmitting the data packet for which it was expecting the acknowledgement. In general, the acknowledgement time-out is fixed and common to all nodes of the network. Since the acknowledgement time-out for data packets travelling a short distance is then the same as for data packets travelling a long distance, the delay of the retransmission is unnecessarily increased for short-travelling data packets, affecting the overall transmission speed of the network. If this disadvantage were addressed by simply reducing the size of the network, scalability would sink further. Therefore, an end-to-end transmission delay of successful data packet delivery as well as transmission delay differences in the network should be minimized.
WO 2009071692 A1 describes a method for characterizing a communication link by considering transmission characteristics of both a MAC layer and a network layer.
EP 1 300 990 B1 describes a method involving transmitting data from a first station via at least a second station to at least one further station. At the interfaces between the stations various data processing requirements are used. The data processing requirements are determined depending on a geographic distance to a defined origin, in particular to a first transmission point. Data processing requirements become less stringent with increasing distance.
KR 2009 0056070 A discloses a method of selecting a relay node by using a competition window in a vehicle ad-hoc network. A source node calculates a competition window including all nodes within transmission range. Each node within the competition window has a message transmission waiting time that is inverse proportional to its distance from the source node. A node whose message transmission waiting time is expired first is selected as a relay node.
U.S. Pat. No. 6,721,537 B1 describes a method for broadcasting a message in an incomplete radio communication network having a fluctuating number of subscribers for forwarding the message. Each subscriber has a transmitting and receiving device for messages and a positioning system for determining its global position. After receiving the message the subscribers determine their own position and the distance from the sender of the message, who is also a subscriber, and transmit the message, with their own position, to further subscribers after a predetermined waiting period, which decreases monotonically as the distance increases.
EP 1 940 089 A1 describes a data transmission method for controlling an arrival delay. A node calculates a cumulative delay of a received packet by using an arrival delay of the packet and a cumulative delay cumulated up to the previous hop. The node then compares the cumulative delay with a target cumulative delay, thereby controlling a transmission profile for the packet so that an expected cumulative delay at the next node becomes closer to a target value. The node writes the cumulative delay in a header of the packet and transmits the packet to the next node using the set transmission profile.
EP 1 764 964 A2 describes a technology that uses a visibility function within a network environment, in particular a vehicular ad-hoc network including a set of nodes. At least one of the nodes can directly transmit to one or more of a subset of the set of nodes. The visibility function characterizes a non-uniform resolution profile within the network environment that extends over at least one node outside the subset of nodes. The sent situation information is conditioned to propagate through the network environment according to the visibility function. The node can also receive situation information that includes a visibility parameter. Once the situation information is received, the node can evaluate the visibility parameter to determine whether the situation information is eligible for continued propagation through the network environment. If the situation information is eligible for continued propagation the node then transmits the situation information.
KR 100 832 519 B1 describes a lighting control system using a wireless tag provided to control a lighting group according to a user position by sensing a lighting control signal of the wireless tag through a second wireless switch and transmitting the signal from the second wireless switch to a first wireless switch through an ad-hoc network.